1. Field of the Invention
The present invention is related to transmission cables and, more particularly, is directed towards a multi-conductor transmission cable whose EMF properties may be precisely controlled, and particularly with respect to such cables intended for use in high speed communication systems and telephone systems.
2. Description of the Prior Art
It is well known that an electric current flowing through a conductor creates an electromagnetic field surrounding the conductor. The surrounding field can, in turn, induce a smaller electric current on other conductors located nearby. The induced current may either increase or decrease the signal magnitude on the adjacent conductor, and therefore can lead to signal errors.
Accordingly, signal bearing conductors are frequently insulated with a low loss material such as, for example, Teflon, which, because of its good dielectric properties, causes the electromagnetic field (EMF) of the conductor to cover a smaller area, thereby reducing the induced current effect of the insulated conductor.
In many communication systems, a conductor pair, known as a send conductor and a return conductor, are required for each signal to serve as either transmission verification or in order to provide system feedback. A common construction of conductor pairs utilizes two individually insulated conductors twisted together in such a fashion so that their respective EMF's are intended to largely cancel one another. In a large transmission cable, many sets of twisted pairs are aligned in a single plane between a pair of outer layers of usually laminated insulation.
A flat transmission cable configuration as above-described suffers from the deficiency that it is impossible to maintain intimate contact between the outer longitudinal layers of insulation and the individual insulations of the twisted pair of conductors. Air pockets are thereby trapped and, as the EMF travels through the air transition zones, the tendency is to distort the signal transmitted on the conductors which can lead to signal errors. Since the twisted insulated conductors vary in their center-to-center distance, the EMF cancellations also fluctuate.
To overcome the foregoing deficiencies, it is quite well known to replace twisted conductors pairs with substantially parllel multi-conductor cables in which the conductors are totally encapsulated in a substantially homogeneous low loss insulation material. While eliminating the problem of signal distortion resulting from trapped air zones, most of the presently available flat cable designs still suffer from one or more disadvantages.
One of the disadvantages of present flat cable designs still results from uncontrollable EMF interference between adjacent conductors. Despite the elimination of the air pocket problem, control of EMF interference remains difficult.
Further, with the advent of faster computer speeds, higher data transmission rates between computer components and peripherals are required so as to minimize delays caused by waiting for information transfer. Another general problem, therefore, with presently available multi-conductor cables is their slow velocity of propagation rates. Present day cables also fail to make any provision for different signal transmission speeds within a single cable.
A further deficiency relates to excessive cost of manufacturing such cables. The extremely low loss, low dielectric constant, high velocity of propagation insulation material is relatively expensive compared to the more lossy, low velocity of propagation polymers. An efficient multiconductor cable design would therefore utilize the low dielectric constant material to the minimum extent necessary to achieve the desired cable characteristics. It may be appreciated that in mass production of such cables, if it were possible to replace even a small amount of the low dielectric constant material with a higher dielectric constant material, tremendous savings in manufacturing costs would be achieved. Many present cable designs, unfortunately, use the expensive polymers unnecessarily and wastefully over the signal conductors as well as the ground conductors.
U.S. Pat. No. 3,763,306 to Marshall exemplifies a multi-layer flat cable design wherein the ground conductors (which do not require a high propagation velocity) are embedded in the same layer and material as the signal conductors. This means that more expensive material with good properties is used around the ground conductors than is necessary, which results in a higher cable cost. Further, the material covering all the conductors has a fixed thickness which can allow uncontrolled EMF interference to bypass the ground conductors and induce false pulses on the adjacent signal conductors.
In U.S. Pat. No. 3,459,879, Gerpheide illustrates a two layer multi-conductor cable construction in which the ground conductors and the signal conductors are embedded in each layer in the same insulating material. Such a construction has the same drawbacks as set forth above with respect to the Marshall design. In addition, in order to eliminate interference, Gerpheide positions the ground conductors of one layer opposite the signal conductors of the other layer to form a triad of ground conductors around each signal conductor. Clearly, the provision of two layers, each with extra conductors, results in a far greater cost than would otherwise be necessary. The construction illustrated in U.S. Pat. No. 3,179,904 to Paulsen is similar.
Multi-conductor transmission line cables are also known which utilize a homogeneous Teflon insulation over both the signal and ground conductors. Such a construction provides a very high propagation velocity, but utilizes the expensive Teflon insulator unnecessarily around the ground conductors.
U.S. Pat. No. 3,735,022 to Estep provides a partial solution to the shortcomings outlined above in teaching a multi-conductor cable design in which signal conductor pairs are first extruded in a low dielectric constant material, such as polyethylene or foam, and the extruded conductor pairs are then extruded once again in a jacket which consists of a lossy dielectric material, such as vinyl. The design of Estep eliminates circumferential air present in prior art twisted pair designs to reduce excess crosstalk, but nevertheless presents several difficulties of its own. Initially, no provision is made in Estep for controlling, to any desired degree, the amount of EMF interference between embedded conductor pairs. Additionally, Estep's design fails to take into account impedance and capacitance effects between adjacent conductors. That is, while it is frequently desirable to reduce cross-interference between conductor pairs as much as possible, other factors and parameters may require designs which permit the amount of EMF interference between the conductor pairs to be varied. Such factors include, for example, the capacitance between the conductors and the impedance of the cable, and are generally a function of relationship between the two conductors to each other, including the amount of insulation contained between them, the dielectric properties of the insulation, the distance between the wires, and the like. In high speed signal communication cables, it is important to be able to achieve the desired capacitance and impedance, while still achieving a certain EMF cancellation.
The Estep construction specifies a conductor insulation having a rectangular, ellipsoid or circular cross-section, while the outer jacket is of generally rectangular cross-section. Such a construction is quite disadvantageous in terms of ease of termination of the cable. The circular, ellipsoid, or rectangular cross-sections contain two or more conductors with no clearly defined individual inner walls between them. As a result, it is extremely difficult to precisely locate and separate one conductor from the other conductor of a pair and obtain a flawless, uniform insulation layer around each conductor. Therefore, perfect connector termination is rarely attained and is very time-consuming to attempt. Further, an imperfectly terminated cable could result in field failures which cannot be detected at the time of termination.
Similar problems arise in connection with telephone cables in which at least one pair of adjacent conductors are normally utilized to carry high voltage. The EMF generated by such high voltage conductors must be controlled in order to prevent interference to adjacent signal-carrying conductors as well as to sensitive electronic components which may be located in close proximity to the terminated end of such a cable.
Other patents of which I am aware which relate to multi-conductor cables include: U.S. Pat. Nos. 2,471,752; 3,219,752; 3,408,453; 3,439,111; 3,576,723; 3,600,500; 3,775,552; 3,800,065; 3,819,848; 3,833,755; and 3,865,972; French Pat. No. 2,036,798; British Pat. No. 1,390,152; and Canadian Pat. No. 697,919.